Question: You say that some food may be haram for some people if it causes them harm or allergic reaction and that it is mubah for others. Can halal or haram vary from person to person?
ANSWER
There are many exceptions. Something halal for someone may be haram for someone else. Examples are as follows:

1. Zakat is fard on the rich while it is not fard on the poor. Similarly, hajj is not fard on everyone. So is the case with qurban and sadaqa-i fitr. (Durar)

As is seen, something may be fard on someone while it is not fard on someone else.

2. It is fard to perform salat in the standing position. However, it is not fard to perform salat in the standing position for the ill. (Jawhara)

As is seen, something may be fard on someone while it is not fard on someone else.

3. Salat does not become fard before its time is on. Salat al-Fajr and Salat al-Isha are not fard in the Polar Regions. However, it is better to perform them. (Tahtawi)

4. The fards of wudu’ are 4. If a person has no feet, then the fards of wudu’ are 3 for him/her. (Halabi)

As is seen, even the fards of wudu’ vary from the healthy to the disabled.

5. It is fard to perform four-rak’at fard salats as four rak’ats. However, performing them as four rak’ats is not fard for travelers. It is wajib for travelers to perform them as two rak’ats. (Ibn Abidin)

6. It is fard on a free lady to clothe her whole body except her face and hands. This does not apply to a jariya. A jariya may uncover her head, her arms, and even the part below her knees. (Hindiyya)

7. It is haram for a man to marry to her sister, but it is not haram to marry to other women. (Bahr-ur-raiq)

8. It is haram for the rich to beg, even to accept zakat. However, there are some exceptions. For example, it is stated in a hadith-i sharif:

(It is permissible to give zakat to a rich person learning 'ilm even if he has forty years’ nafaqa.) [Radd-ul-Mukhtar]
As is seen, something may be halal for a rich person while the same thing is haram for another rich person.

9. Consuming carrion or wine is haram. However, it is not haram for a person who is about to die of hunger or thirst. (Bazzaziyya)

10. Some people have an allergy to some medicines, e.g. penicillin. They may cause even death. It is haram to have these medicines for those who have an allergy to them. However, they are not haram for others. (Bariqa)

Like medicines, some food, vegetables, or plants may cause allergic reactions. If something is bad for someone with a chronic illness, it may not be harmful for the healthy. Fish, milk, eggs, chilies, eggplants, or strawberries may be bad for some people. They are haram or makruh only for those people. They are mubah for those who are not affected negatively. Similarly, opium and poisonous plants are haram in large quantities. However, it is permissible to have them in small quantities for medicinal purposes. (Hadiqa, Zawajir)

Theine present in tea is the same as caffeine present in coffee. There is 2.5 - 3% caffeine in tea and 1.3% in coffee. Caffeine increases memory, improves blood flow, warms the body, stops fatigue, and eases digestion. Much may affect nervous system. Even little coffee may have negative effect in the case of heart diseases, neurasthenic tendencies, and on children. [Food Chemistry, p. 658]

As you see above, what Islamic scholars said is written in medical books, too. That is, caffeine, theine, or nicotine may be harmful for some people, and not for others. They are haram for those who are affected negatively. Saying they are haram for everyone means changing the religion.